Gray glass has found particular utility for architectural applications as building glass and has been considered for automotive glass applications. Glass is generally defined by certain spectral properties like dominant wavelength (color), excitation purity, and light transmission. The lower the excitation purity of a color, the closer it is to being a so-called neutral color which does not distort the hues of objects seen through it.
Those skilled in the art know that dominant wave-length, purity and light transmission all vary unpredictably with one another. Consequently, developing a new glass composition having a particular color, purity and light transmission value is generally difficult. For example, an experimental change in the amount or relative proportions of one or more colorants in a glass composition intended to bring one of these numerical values closer to a target value may cause one or both of the other values to drift off target. Hence, the particular portions of the colorants is critical to developing a particular glass composition.
Numerous different patented grey glasses have been manufactured by using as colorants iron oxide, cobalt oxide, and chromium oxide, selenium or nickel oxide. These same colorants have also been used to make glasses of colors other than grey, as would be appreciated however, at least one or more colorants being in different amounts. For example, a brown or bronze glass can be made from iron oxide, cobalt oxide, and selenium, while the glass can be made blue if the proportion of a component, e.g., cobalt is increased. Hence, proportions of colorants is critical to the spectral properties of the glass.
Considering gray glasses, U.S. Pat. No. 4,104,076 discloses a gray glass composition whose colorants consist essentially of: iron oxide, cobalt oxide, and selenium, where to avoid using nickel oxide, either or both chromium oxide or uranium oxide are employed. Cheng in U.S. Pat. No. 5,278,108, in contrast, forms a grey glass with colorants iron oxide, cobalt oxide, and selenium, specifically avoiding constituents that are disclosed to cause difficulty in manufacturing, i.e., chromium and manganese. In U.S. Pat. No. 4,873,206, a dark gray nickel-free glass is manufactured using iron oxide, cobalt oxide, and selenium, therein it is disclosed that the composition is to be essentially free of chromium, titanium, and manganese. Another approach is taken in U.S. Pat. No. 5,346,867, wherein the neutral gray glass is made using the colorants iron oxide, cobalt, selenium, manganese oxide, and optionally titanium oxide. Still another glass is taught in U.S. Patent which uses cerium oxide, iron oxide, selenium, and optionally cobalt oxide, nickel oxide and titanium oxide to obtain a gray color. U.S. Pat. No. 5,656,560 discloses bronze-tinted or grey-tinted glass whose color is obtained by using at least a relatively high amount of manganese oxide, and optionally any of iron oxide, vanadium oxide, nickel oxide, copper oxide, and cobalt oxide. One taught object of this patent is to form its gray or bronze glass without the use of selenium, a very volatile and costly colorant. This last patent, again demonstrates the unpredictable nature of the glass coloring art, where using e.g., iron, manganese, nickel and cobalt results in several example embodiments of a bronze-colored glass.
In the present invention we have developed a new medium grey glass composition, different from those disclosed above, which contains in specified proportions iron oxide, manganese oxide, cobalt oxide, and selenium which provides a glass with excellent ultraviolet (UV) and infrared (IR) absorbing properties and very low excitation purity. This low excitation purity makes the glass ideally suited for both automotive and architectural application since it has a neutral appearance favored by designers. Its excellent UV and IR absorbing properties make it ideally suitable to prevent color fade damage to components inside a car or building and to keep the interiors cool. As would be appreciated, the UV and IR light absorption properties are especially valuable when the glass is for buildings since when heat is absorbed by the glass, the load on building air conditioners is reduced. Therefore, developing a glass with these spectral properties is very important.
Iron oxide exists in two forms in the glass melt, the oxidized form of iron oxide (Fe.sub.2 O.sub.3) absorbs UV light and the reduced form of iron oxide (FeO) absorbs infra red light. Thus these two forms of iron oxide in the glass lower the UV and IR transmittance through the glass products. Adding iron oxide to a soda-lime-silica glass under normal furnace conditions improves both the UV and the infrared absorption of the glass since the concentration of the iron forms is correspondingly increased. This improvement is at the expense of visible transmittance, however. That is, as iron oxide is added the color of the glass darkens so that the visible transmittance is correspondingly decreased. In the present invention, the incorporation of the particular colorants in combination has been found to allow for improved UV and IR absorbing properties when adding more iron oxide without a commensurate darkening of the glass color. Hence, it retains good visible transmission properties.